At present, essentially three different approaches exist for realizing such a protection system for circular-saw benches and format circular saws which are intended to prevent a user from coming into contact with the rotating saw blade or to suffer a serious cutting injury.
The protection system of an American manufacturer, carried and marketed under the trade name Saw Stop Inc., is an emergency braking system which permits braking of the machine tool on the basis of a direct engagement of a brake actuator system with the saw blade, as soon as a suitably designed sensor has detected a dangerous situation. In this case, by the use of a hot-wire trip element, a rotatably-mounted aluminum block is pushed with the aid of a preloaded spring into the toothing of the operating saw blade, the aluminum block wedging itself therein, and in this manner absorbing the rotational energy of all geometries of the machine tool rotating during the sawing process. As a secondary effect, this one-sided application of force on the cutting is used to sink the saw blade into the saw bench by way of a structurally specially-designed suspension device disposed on the saw bench. With the aid of this setup, it is possible to avert serious bodily injury to the machine-tool operator triggering the protective mechanism. The disadvantage in this case is the direct attack on the working tool, thus, on the cutting geometry of the saw blade, since the breakaway of parts of the toothing creates additional danger potential for the operator. In addition, in order to restore the preparedness of the protection system, it is necessary to replace the brake unit including the saw blade by a replacement unit ready for use, which the operator must stock up, in order to be able to continue working utilizing the protection function after a braking action has taken place. This is associated with considerable resultant costs and suitable expenditure of time for procurement and installation. In addition, it may be assumed that all component parts affected by the braking process, that is, all rotating geometries of the machine tool, are subject to great stress during the deceleration phase. Neither the manufacturer nor relevant publications concerning this system have data with regard to the long-term endurance of the device.
A further approach pursues the use of a protection system exclusively for sinking the saw blade into the saw bench without initiating a procedure for braking the saw blade. In this case, with the aid of a pyrotechnic priming charger, the saw blade, including the main shaft and its bearing, is moved out of the danger zone, so that serious injuries to the operator may be prevented. The disadvantage in this type of protection system is the necessity of moving relatively large masses in the form of assemblages to be lowered, under strict time requirements in the millisecond range. Furthermore, the pyrotechnic igniters thereby necessary, which prove to be imperative for this protection actuator system, give rise to a costly part reversibility which limits, both timewise and organizationally, an immediate continuation of work on the machine tool with a protection system ready for use. Furthermore, for reasons of limited space and very specific procedure, this protection system is only suitable for larger stationary devices like, for example, circular-saw benches which permit such a structure in their interior. However, this system has to be ruled out for use on smaller, hand-guided devices such as cross-cut and miter saws or panel saws.
A disclosure from a publicly sponsored project by the name of “Cut-Stop” (VDI/VDE/IT) with respect to a protection system for format circular saws by the Institut für Werkzeugmaschinen (IFW) (Institute for Machine Tools) of the University of Stuttgart discusses an approach which, with the aid of a special form of a disk brake system, namely, a self-amplifying wedge brake, brings the main shaft of the machine tool and therefore the saw blade to a standstill. In this case, a wedge is accelerated with the aid of a pyrotechnic igniter and subsequently pushed between a stationary wedge guide in the form of a modified brake caliper and the rotating brake disk. For the selection and combination of specific wedge angles α and brake-lining values μ, the system works in self-locking fashion, so that the specific time demands on the braking procedure may be fulfilled with this design as a function of the mass inertia to be decelerated. However, the disadvantage of this protection system lies in the fact that in this case as well, as discussed in the cited publication, subsequent to the triggering of the protection actuator system, it is necessary to replace the complete braking unit because of the wedge jammed in the friction pairing. The expenditure of time needed for the scope of work to be carried out amounts to approximately 10-12 minutes for the complete restoration of the system preparedness. Therefore, a partial restriction of the immediate system reversibility holds true here, as well.
German patent document DE 195 36 995 A1 discusses a fail-safe brake for elevators which has a device that, in the event a predefined maximum speed of the means of transport is exceeded, brakes the drive with a speed-dependent deceleration and possibly also stops it. The fail-safe brake of DE 195 36 995 A1 acts directly, as a function of rotational speed, on the driving disk of the cable-operated conveying system and is able to limit its rotational speed. The fail-safe brake in the form of a centrifugal brake additionally has a device for amplifying the braking force as a function of the conveying speed.